1. Field of the Invention
The present invention relates to a decoder device for use in a receiver for decoding packet signals that include data codes and control codes.
2. Description of Related Art
The IEEE-802.11a standard requires that a header of a packet signal should include control codes containing information for decoding the packet signal. The decoding information consists of a modulation formula (QPSK, BPSK, 16-QAM or 64-QAM) and a coding rate (½ or ¾) of an error correction code. The packet signals received by a receiver are divided into data codes and control codes, and the control codes are first decoded and analyzed to determine which one of the modulation formulae and which one of the coding rates are specified in the control codes as the decoding information. Since it is predetermined that the control codes themselves are coded under the modulation formula BPSK and the coding rate ½, the control codes can be decoded based on the BPSK and the coding rate ½. After the decoding information is detected, the data codes are decoded based on the detected decoding information.
The decoding is performed using a Viterbi decoder. Since the decoding process performed by the V-terbi decoder is relatively slow, it takes a long time to process the packet signal as a whole, if the data codes are decoded based on the decoding information after it is obtained by decoding and analyzing the control codes. A real time decoding is not possible.